1. Field of the Invention
This invention relates generally to structures and devices produced by techniques of nanotechnology.
More specifically, the invention relates to such structures and devices incorporating at least one element, essentially in one-dimensional form, and that is of nanometer dimensions in its width or diameter, and that preferably is produced by the so-called Vapor-Liquid-Solid (VLS) mechanism. For the purposes of this specification, such element will be termed a “nanowhisker”.
2. Brief Description of the Prior Art
Nanotechnology covers various fields, including that of nanoengineering, which may be regarded as the practice of engineering on the nanoscale. This may result in structures ranging in size from small devices of atomic dimensions, to much larger scale structures for example on the microscopic scale. Commonly, such structures include nanostructures. In certain contexts nanostructures are considered to be those having at least two dimensions not greater than about 100 nm, with some authors using the term to identify structures having at least two dimensions not greater than about 200 nm. Nevertheless, some procedures for fabricating such small structures are useful for structures having at least two dimensions somewhat greater, e.g., structures having at least two dimensions not greater than about 1 micrometer (μm). Ordinarily, layered structures or stock materials having one or more layers with a thickness less than 1 μm are not considered to be nanostructures. Thus, although the term “nanostructure” is more classically considered to refer to structures having at least two dimensions not greater than about 100 nm, in the following discussion, the term “nanostructure”, “nanowhisker”, or “nanoelement” is intended to include a structure having at least two dimensions not greater than about 1 μm.
Nanostructures include so-called one-dimensional nanoelements, essentially in one-dimensional form, that are of nanometer dimensions in their width or diameter, and that are commonly known as nanowhiskers, nanorods, nanowires, nanotubes, etc.
As regards nanowhiskers, the basic process of whisker formation on substrates, by the so-called VLS (vapor-liquid-solid) mechanism, is well known. A particle of a catalytic material, usually gold, for example, on a substrate is heated in the presence of certain gases to form a melt. A pillar forms under the melt, and the melt rises up on top of the pillar. The result is a whisker of a desired material with the solidified particle melt positioned on top. (See E.I Givargizov, Current Topics in Materials Science, Vol. 1, pages 79-145, North Holland Publishing Company, 1978.) The dimensions of such whiskers were in the micrometer range.
Although the growth of nanowhiskers catalyzed by the presence of a catalytic particle at the tip of the growing whisker has conventionally been referred to as the VLS (Vapor-Liquid-Solid) process, it has come to be recognized that the catalytic particle may not have to be in the liquid state to function as an effective catalyst for whisker growth. At least some evidence suggests that material for forming the whisker can reach the particle-whisker interface and contribute to the growing whisker even if the catalytic particle is at a temperature below its melting point and presumably in the solid state. Under such conditions, the growth material, e.g., atoms that are added to the tip of the whisker as it grows, may be able to diffuse through a the body of a solid catalytic particle or may even diffuse along the surface of the solid catalytic particle to the growing tip of the whisker at the growing temperature. Evidently, the overall effect is the same, i.e., elongation of the whisker catalyzed by the catalytic particle, whatever the exact mechanism may be under particular circumstances of temperature, catalytic particle composition, intended composition of the whisker, or other conditions relevant to whisker growth. For purposes of this application, the term “VLS process”, “VLS method”, or “VLS mechanism”, or equivalent terminology, is intended to include all such catalyzed procedures wherein nanowhisker growth is catalyzed by a particle, liquid or solid, in contact with the growing tip of the nanowhisker.
International Application Publication No. WO 01/84238 discloses in FIGS. 15 and 16 a method of forming nanowhiskers wherein nanometer sized particles from an aerosol are deposited on a substrate and these particles are used as seeds to create filaments or nanowhiskers.
For the purposes of this specification the term nanowhiskers is intended to mean “one-dimensional” nanoelements with a width or diameter (or, generally, a cross-dimension) of nanometer size, the elements having been formed by the so-called VLS mechanism. Nanowhiskers are also referred to in the art as “nanowires” or, in context, simply as “wires”, and such terminology, as used in this application, is equivalent to the term “nanowhiskers”.
Several experimental studies on the growth of nanowhiskers have been made, including those reported by Hiruma et al. They grew III-V nano-whiskers on III-V substrates in a metal organic chemical vapor deposition (MOCVD) growth system. (See K. Hiruma, et al., J. Appl. Phys. 74, page 3162 (1993); K. Hiruma, et al., J. Appl. Phys. 77, page 447 (1995); K. Hiruma, et al., IEICE Trans. Electron. E77C, page 1420 (1994); K. Hiruma, et al., J. Crystal Growth 163, pages 226-231 (1996)).
Hiruma et al. have made pn junctions within nanowhiskers by doping GaAs whiskers with Si using Si2H6, during the growth process, and switching the dopant to an opposite conductivity type (carbon) during growth: K. Hiruma et al., J. Appl. Phys. 77(2), 15 Jan. 1995 p.447, see pages 459-461; and K. Hiruma et al J. Appl. Phys. 75(8) 4220 (1994). In general, there are problems in that the definition of the junction within the nanowire is not good enough for electrical components, and in that the presence of dopant ions within the crystal creates crystal imperfections and reduces carrier mobility.
In another approach by Lieber et al, WO-A-03/005450, nanowires were produced, different wires being doped with opposite conductivity type dopants, and two wires of opposite conductivity type were physically crossed, one on top of the other, so that a pn junction was formed at their point of contact. A difficulty with this approach is the extra step required of physically positioning the nanowires.
In planar semiconductor processing, various doping techniques are known. One technique that is valuable with heterojunctions is known as modulation doping. In this technique, carriers from a doped layer of, e.g., AlGaAs, diffuse across an interface with an undoped material, e.g., GaAs, and form a very thin layer of carriers of very high mobility, within a potential well, next to the interface—see for example FIG. 1 of WO 02/19436.
U.S. Pat. No. 5,362,972 discloses an FET wherein the current flowpath between source and drain is composed of GaAs nanowhiskers. The nanowhiskers are surrounded by n-doped AlGaAs, to create by modulation doping a one-dimensional electronic gas within each nanowhisker.
WO 02/020820 discloses a modulation doping technique in Coaxial Heterostructure Nanowires, wherein dopants in an outer coaxial layer donate free carriers to an inner nanowire.
In other techniques, doping of a semiconductor region in a planar semiconductor device occurs by diffusion of ions from an adjacent region of polymer; see Guk et al., Semiconductors Vol. 33(3), pp. 265-275, March 1999.
In co-pending U.S. patent application Ser. No. 10/613,071, filed on Jul. 7, 2003, in the names of Samuelson and Ohlsson, the contents of which are incorporated herein by reference, a process was disclosed for producing nanowhiskers, and structures were disclosed incorporating nanowhiskers.